[NP] MAC-DGS Method for Navier-Stokes Equation

MAC-DGS theoretical outline. Brief intro: MAC (Marker and Cell) is a special structure designed for finite difference method solving Stokes equation, while DGS (Distributive Gauss Seidel) is a kind of special algorithm designed as a proper smoother for Stokes equation. New difficulties raised from Poisson equation to Stokes equation, like discrete inf-sup condition should be satisfied for convergence, more complex operator with no diagonal dominant should be considered instead of simply Laplacian operator, etc., and that's why we replace traditional Poisson finite difference structure by MAC, as well as replace traditional smoother (Gauss Seidel) by DGS.